Groundspeed/60 = miles per minute
Altitude to lose/descent rate = time to lose that altitude

Then just multiply your miles per minute by the number of minutes to lose your altitude and boom.
(300/60 = 5 miles per minute; 27,000 to 17,000 = 10,000; descend at 2000fpm = 5 minutes to lose the altitude; so you will need 25nm to descend)